If the seas are truly rough a cabin closest to the waterline is best for having the least amount of motion. Thus if it is a problem for you, look at booking something in the middle of the lowest passenger deck - not too far forward, or to a lesser extent not too far aft. Actually something with a window is better since seeing the horizon can reduce the effect of motion sickness. An inside cabin gives no point of reference and can add to the bad feeling with claustrophobia.